Random noise system for vibration testing



Dec. 29, 1964 c. THEODORE ETAL 3,163,809

RANDOM NOISE SYSTEM FOR VIBRATION TESTING Filed Jan. 10. 1961 2 Sheets-Sheet 1 20 L ELECTRO- FILTER ?OWER AMPLIFIER MECHANICAL E I 2| TRANSDUCER I.

MECHANO- 27-ELECTR1CAL TRANSDUCE .L PHOTO- 55 MULTIPLIER PHOTO- MULTIPLIER ATTEN- RECTIFIER- z FI TER uATo METER TT N- RECTIFIER- GATE METg R FILQIER I I I 1 l FIG. 1.

ATTEN- FILTER FILTER FILTER U lO N 2 IN VENTORS CHARLES THEODORE JAMES A. ROSS AGENT FREQUENCY a- Dec. 29, 1964 c. THEODORE ETAL. 3,153,809

RANDOM nozsr: SYSTEM FOR VIBRATION wss'rmc Filed Jan 10. 1961 2 Sheets-Sheet 2 2g 2; 67 w: POWER I ELECTR- AMPLIFIER MECHAN'CAL es TRANSDUCER] MECHANO- ea 27. ELECTRICAL TRANSDUCER A I 2 l l -ATTEN- UATOR FILTER I 3 3 I L ATTEN @UATOR I n I r I RECTIFIER- FILTER T ATTENlIJ ATOR *METEB *4 REcTIFIER- A FILTER ATTENUATOR METER l I 2 2 2 I REcTIFIER- FILTER ATTENUQTOR T METER REcTIFIER- FILTER ATTENu IToR METER 4 i I i In i F I G. 4.

IF. -eo NOISE -FILTER MIXER |.F. 7 NOISE FILTER OSCIL- 2 2 LATOR '55 INVENTORS l.F. CHARLES THEODORE NOISE L. FILTER JAMES A. Ross F BY 34. fm

AGENT United States Patent C 3,163,309 RANDUM NQESE Slfd'ilEli i EUR Vl'lBRATliQN TESTING Charles Theodore, lisos Angeles, and llamas A. Ross,

range, Caliifi, assignors to Ling-Teinco-Vought, inc,

a corporation of Delaware Filed fan. in, Edit, Ser. No. 31,731 is @laims. (ct. sis-r32 Our invention relates to electrical systems and particularly to such systems as include electromechanical elements operated in a random manner.

In present technology numerous processes are carried out by employing a noise type of exciting signal. The efficacy of such operations are often related to the degree of complete randomness of the noise signal; the employment of pure Gaussian noise.

Important among these processes is that of vibration testing under controlled conditions, as is accomplished with the known electromechanical transducer, or shaker, in combination with an electrical energizing system. Random noise constitutes an important type of test signal in this art and the capability to retain such a signal completely random with a specified amplitude vs. frequency characteristic regardless of shaker and fixture resonances and regardless of all other spurious influences is an important one. Resonances and spurious effects can be prevented from influencing the performance of the system if adjustable filter means are provided that eliminate peaks and valleys in the amplitude vs. frequency characteristics of the system by providing inverse portions of this characteristic.

Further, many specific types of vibration can be synthesized from a random noise source by employing an adjustable filter complex to give the system an amplitude vs. frequency characteristic that characterizes the specific type of vibration; for instance the type of vibration encountered in shipment by railway freight or by truck.

We have formed these systems and have employed them for both the purely random and the synthesized types of vibration testing. As might be expected, we employed one random noise source. We did not then see any reason for more than one such source. The spurious resonances were removed and/or the specific types of vibration were synthesized by using many paralleled filters, perhaps twenty to one hundred. Each of these was adjustable in attenuation and each pass a specific band of the whole frequency spectrum of interest, so that in the whole the complete frequency band was passed.

it will be understood that the filters were formed to pass frequency bands adjacent one to the other, so that all frequencies could be passed and so that by adjustment of attenuation associated with any filter the amplitude characteristic of the whole passed band could be given any desired characteristic. It will also be understood that no practical filters exist with infinitely sharp cut-off frequencies. All have sloping skirts, and so adjacent filters had common frequencies in the joint passed band.

We found, unfortunately, in this initial apparatus, that the inescapable phase shift of any signal occupying the common frequencies caused the contribution to the signal from one filter to be phased with respect to that of the adjacent filter so that either a cancellation or a re inforccmcnt of the signal amplitude resulted at that frequency. While this might have been compensated for with non-adjustable filters, the essential requirement of the system called for full adjustability of all filters. We were thus introducing sharp cuts or peaks because of the cancellation or reinforcement phenomena by the very means that were required to eliminate these effects from the system caused by other elements of it.

We then discovered that the peaks and valleys were caused by coherence of the random noise as it passed through adjacent filters at common frequencies. Because there was only one noise source the signal energy components at any instant of time were derived from a single electrical energy variation. Accordingly, at the frequencies common to both of each adjacent filters, component energy was present from both filters. Because of the inescapable phase change, or phase distortion, occurring at the skirts of all known electrical filters, the resulting combined amplitude was either greater or smaller than normal, depending upon the particular phase characteristics involved.

We were able to completely remove this serious limitation by providing different sources of noise for adjacent filters. The noise signal components present in one filter were then statistically different from the components present in the adjacent filter at any given instant of time. In fact, the desired complete randomness of the noise signals perfects this situation.

We have embodied this novel method in two general arrangements of apparatus. In one we employ a separate noise source for each filter channel. In the other we employ at least two noise sources and connect these alternately to filters having adjacent pass bands. In either arrangement it is seen that We accomplish the required filtcr-to-fllter randomness.

in addition to manual attenuation control means for setting the overall desired characteristic we have provided novel feedback means for establishing servo control, i.e., automatic gain control, by which the contribution of each filter channel is maintained at a selected amplitude. For this reason, Whether we employ a separate noise source for each filter channel, or the minimum of two noise sources, the result as to simplicity of apparatus and economy in manufacture allows a choice, rather than dictating the apparent simplicity of two noise sources.

For the automatic gain control a feedback servo element is required for each filter channel. With a preferred type of noise source according to our invention, the servo element is a simple component of that noise source and so overall simplicity is obtained. Specifically, a photomultiplier photoelectric cell is employed as the random noise source and a simple incandescent lamp, terminating the feedback loop, determines the noise signal level output from the photomultiplier. For the two noise source embodiment we prefer to employ a photoelectri- .cally sensitive resistor as a gain control in association with an incandescent lamp as the terminating element of the feedback loop. Alternate arrangements are also possible, as will be later detailed.

An object of our invention is to provide a random noise actuated electrical system.

Another object is to provide an adjustable random noise electromechanical system.

Another object is to provide an electrically controlled vibration system having a selectable amplitude vs. frequency characteristic.

Another object is to provide an electromechanical sys tem having a selectable amplitude vs. frequency characteristic devoid of spurious responses engendered by the selection of said characteristic.

Another object is to provide a multiple channel electromechanical system having simple automatic gain controlled random noise sources. I

Another object is to provide a multiple channel clectromechanical system having only two noise sources to provide completely random operation.

Another object is to provide a multiple channel electromechanical system having heterodyne means for accomplishing frequency band filtration in combination with plural noise sources.

Other objects will become apparent upon reading the following detailed specification and upon examining the acompanying drawings, in which are set forth by way of illustration and example certain embodiments of our invention.

FIG. 1 shows the attenuation vs. frequency characteristics of a plurality of filters required for our system.

FIG. 2 shows an embodiment of our system which employs a separate source of noise for each channel thereof,

FIG. 3 shows another embodiment of our system which employs a plurality of sources of noise signal typically much fewer than the number of channels in the system, and

FIG. 4 shows still another embodiment of our system in which filtration is at frequencies higher than the frequencies utilized.

In FIG. 1 is shown the family of filter characteristics required to obtain a wide band electrical system in which the amplitude vs. frequency characteristic of the whole system may be altered as desired. The individual frequency characteristics are preferably of the band pass type, as identified by filter 1, filter 2, filter 3, and this continues with increasing frequency to filter n, where n is the number of frequency bands which may be individually adjusted.

The characteristics drawn in solid lines represent the attenuation of each filter for a uniform overall characteristic of attenuation as a function of frequency; the known flat characteristic. Onthe other hand, the solid line characteristics for filters l and 3 combined with the dotted line characteristic for filter 2 represent an electrical characteristic with reduced attenuation over the passed band of filter 2. This may compensate for the lack of response of an electromechanical transducer over this frequency band; such a transducer being a loudspeaker, a shaker unit for vibration testing, or the like. It is desirable that the attenuation of any of the filters be smoothly variable over an amplitude range of many decibels so that any and all idiosyncrasies of the system can be compensated for.

As has been previously brought out, the frequencies common to adjacent filters change when such adjustments are made. This is easily seen in FIG. 1. It is very well known that the skirts of practically realizable filters cannot be infinitely sharp; that is, represented by vertical lines in FIG. 1. A typical practical relation is illustrated in that figure. It will be noted that when the attenuation in the circuit channel of filter 2 is the same as for the other filters (the solid lines) the crossover frequencies are at 4 and at 5 for the adjacent filters on the low and the high frequency sides of filter 2. However, when the attenuation of filter 2 is reduced, so as to be represented by the dotted curve 6, these cross-over frequencies have changed; being new at points 7 and 3. These frequencies turn out otherwise to be closer together, but the fact that there is a change is the significant effect.

it is for this reason that a shift in the phase on the skirts of the frequency-adjacent filters produces such I marked amplitude irregularities when the random signal feeding each of the filters is the same and remains related for all periods of time.

On the other hand, when frequency-adjacent filters are fed from different sources of random electrical variations, it is seen that the effects of phase distortion at the skirts has noefiect. If the two sources feeding adjacent filters provide completely random signals the probability is that there will not be frequency components passing through adjacent frequency filters at common frequencies. Accordingly, there is no algebraic addition of amplitudes and no distortion of the amplitude characteristic because of phase distortion at the shirts.

In FIG. 2 complete randomness is attained by employing a separate electrical noise source for each filter chan- 6;. nel. While it might be considered that this was an ex 'pensive Way of achieving the objective, it is seen that this is not so when it is understood that each source is relatively inexpensive and that each includes a simple and effective means to accomplish automatic gain control for each filter channel.

Such a noise source is shown generally at 10 in FIG. 2. It is comprised of a light-tight enclosure, such as indicated at 19, the photomultiplier proper ill, the illuminating lamp 112 and other elements auxiliary to the photomultiplier.

ince the purpose of the photomultiplier is to act as a noise source and not to amplify a specific light signal in the conventional manner, the relatively poor devices of this class may be employed. A high signal to noise ratio is not important because only the noise is allowed to be produced. The extremely low noise level often sought in the absence of illumination in other applications is also not essential, since the level of residual noise signal present is in all cases sufiiciently low to provide the desired dynamic range of noise signal as we use the device.

The emission of electrons by a photoelectric surface in response to incident photons is a random phenomenon and is the basic function controlled by the relatively constant (but servo adjustable) level of illumination by lamp 12. Residual effects include spontaneous electron emission from the photo cathode, electron emission caused by ion bombardment of the cathode by ionized residual gas, and electrostatic field emission from sharp edges because of the high voltage gradient existing there. However, all of these effects are also random and so the preferred random noise signal is obtained.

in operation, the light from lamp 12 impinges upon photo cathode 13 by close juxtaposition as is usual be tween such elements. The illumination from the lamp is constant unless a gain adjustment of the whole feedback loop is being made. Consequently, no alternating signal of light flux is impressed upon the cathode and so only random noise is produced. The amplitude of this noise is essentially a linear function of the amount of light falling upon the cathode.

in the known fashion, the few electrons emitted by cathode 13 are multiplied in the electron multiplier comprised of dynodes 14, of which nine are shown in FIG. 2. The dynodes are connected by a series of resistors generally indicated at 15 and extending from a negative potential source indicated at 16, to ground. Capacitors 17 are conventional for enhancing electrical stability. The electrical noise output signal appears at anode l3, greatly amplified with respect to the initial level at the photo cathode. The anode circuit is completed through resistor 19, Which has a typical value of one megohm or less. This signal passes to amplifier A the output level of which is a few volts.

It has been mentioned that our system may have from 20 to filter channels covering a frequency range of a few thousand cycles in a continuous spectrum in typical embodiments. In FIG. 2 filter l is the lowest frequency filter of the group; It may have a central frequency of the order of twenty cycle and a passed band of the order of this amount. It will be understood that the matters of frequency range, width of filter band pass and type of filter are subject to wide variations within the purview of our invention.

In FIG. 2 resistor 29 is connected to the signal output terminal of filter l as the element connecting that filter to a resistive matrix for combining all increments of the whole frequency spectrum for driving the electromechanical transducer. In certain parts of the figures the elements are connected by only one line. In the case of these block representations a ground return path is to be understood. The output impedance of filter 1 is of the order of thousands of ohms and resistor Ztl has approximately the same value. Shunt resistor 21 essen- E5 tially matches the combined impedance of all of the resistive outputs from the several filters and so has a typical value of less than one hundred ohms.

According to the usual and necessary practice of audio sound or of vibration testing a power amplifier 22 is connected to the output of the resistive matrix in order to suitably drive the electromechanical transducer 23. This amplifier may have a power capability of from a fraction of 21 watt to the order of megawatts, depending upon the practical application.

Electromechanical transducer is typical y an electromagnctically driven device, such as a cone type loudspeaker or any of the electromagnetically driven vibratory shakers employed in the vibration testing art, as well as sonar devices of this class. Other types of useful loads may also be employed. Our invention is primarily concerned with operating electrical systems in a truly random manner according to a controllable characteristic and the particular instrumentalities employed may vary over a wide range.

In a manner analogous to the channel already described, a second channel including photomultiplier 2, amplifier A .lter 2 and matrix resistor 24 is provided to contribute a second passed band of electrical noise energy adjacent to the passed band of the first filter channel. These two passed bands have frequencies in common, as

shown in FIG. 1, and has been previously described, but

the noise signals are completely random because a completely separate noise source is employed for each channel. Typically, the second channel may have a central frequency of the order of forty cycles andthe filter may be of the same type as filter 1.

Still further, a third channel, including photomultiplier 3, amplifier A filter 3, and matrix resistor 25 provides a channel adjacent to channel 2. The central frequency of channel 3 may be fifty-five cycles.

The dotted vertical lines under the major components of these channels and the letter 11 indicates a continuation of this adjacent channel structure to 11 channels, which in the sum embrace the whole frequency spectrum of interest. As has been inferred, n may have a relatively high value, such as form 20 to 109.

For all of the direct channel filters ll, 2, 3, II, we have employed a passive type filter comprised of a series of several pi sections of the M derived type in order to obtain individual filters having relatively steep skirts and adjacent bands of frequencies. Flier l, of course, need be only a low pass filter.

We now turn to the feedback part of the embodiment of FIG. 2. p i

lviechanoelectrical transducer 27 is mechanically connected to electromechanicaltransducer 23 by a connection 28. Transducer 2"? may be an accelerometer, a rigid transducer constituted to transmit force, orother type of motion to electrical response device. It may be mounted directly upon the table of transducer 23, ete.,.as will be understood.

The electrical output of transducer 27 is amplified as necessary to drive the lamp load of each channel, as lamp 12, but an illustration of such an amplifier in detail has not been shown. In fact, certain of the filters of the feedback channels contain amplifying vacuum tubes, as

is brought out below.

Considering channel 1, filter 1 contains active elements Eli and 31; vacuum tubes or the equivalent. As shown,

vacuum tube 3t? is a pentode, the control grid of which is connected to the electrical output terminal of mechanoelectrical transducer 4.7. Grid return resistor 32, screen dropping resistor 33 and anode resistor 34 have the usual values appropriate to the vacuum tube employed. A coupling capacitor 35 connects the plate (or anode) of tube to the grid of triode 31. Connected from this grid to ground is the parallel resonant circuit composed of U inductor 36 and capacitor 3 This is the frequencydiscriminative element of this filter and we have founo this configuration desirable for the low frequency filter units.

A. common cathode connection is employed between tubes 30 and 31, in which resistor 33 is common to both cathodes and has a resistance value of less than one thousand ohms and resistor 3h connects between the cathodes but carries only the cathode current of tube 31. Resister 39 has a typical resistance value of everal thousand ohms. This circuit arrangement gives positive feedback to increase the resonant Q of the tuned circuit 36$? The electrical noise energy within the narrow frequency band passed by filter 1 passes from the cathode of vacuum tube 31 to amplifier A which amplifier raises the signal level to several volts for subsequent rectification by bridge rectifier 41 in the rectifier-meter entity 1. The input to the rectifier is conveyed from. the amplifier to opposite sides of the bridge by two capacitors 4-4, each having a capacitance of the order of a tenth microfarad.

Capacitors and 46 are connected to the output of the rectifier and constitute the capacitance part of a low pass filter. The capacitance values of these capacitors are in the multi-microfarad range in order to accomplish reasonably complete filtering and most importantly to provide a necessary integrating efiect. The latter eject is required to provide a reliable reading or" an indicating meter to be described. Because of random nature of the noise signal, the energy of the rrequency band conponents vary from instant to instant for the relatively narrow bands which we analyze. Only a reading representing the integration of the noise signal over a period of several seconds duration can be relied upon as being the measure of the long time, level in the given frequency band.

Resistor 4 7 connects between oneterminal or each of capacitors i5 and as and forms part of the resistive portion of the low pass filter. it has a resistance of a few thousand ohms. Resistor as connects to resistor but is only in circuit with meter Resistor 555 has a resistance value twice that of resistor 4-7. Meter 4% is a milliammeter. It reads the noise signal level in the particular filter band pass for the output from ElBCllOlllfiCllElll ical transducer 23 as determined by mechanoelectrical transducer 27', irrespective of the setting of the feedback attentuation next to be described.

The attentuator per se is potentiometer Ell, which has a total resistance of the order of 500 ohms. This is connected to lamp 123 i photomultiplier l to complete the feedback loop. Certain auxiliary elements are also included. Lamp 12 is primarily powered by battery 53, the circuit thereto being completed through resistor El,

which has a resistance of one hundred ohms in a typical embodiment. The voltage of batt ry is of the order of 26 volts and in practice is an AC. to DC. filtered power supply.

The adjustable arm of potentiometer is connected to the lamp side of resistor $11 through Zener diode The latter has a constant voltage (breakdown) of several volts and acts to provide a delayed AWE. to the feedback path. The rectifier output, arising in potentiometer 50 is poled to reduce the efiective voltage on lamp l2; i.e., with no rectifier output the lamp glows brightly, while with large rectifier output it glows dimly. Until the voltage breakdown of the Zener diode is exceeded no output of the rectifier is applied to reduce the bri iancy of the lamp, thus providing the delayed type of automatic volume control (AWE).

A differential amplifier may be connected between the top ofresistor 51 and the top (negative) terminal of be tery 53. Tll's is employed to increase the sensiti y of the lamp circuit in such embodiments this may desired- I It is to be understood that although the time constant filtering of filter ii, as, 4'7, etc. is necessary and desirable with normally employed narrow band filters for spectrum analyzing, other types of operation may be v V Z employed. If the filters are broad band, or it is desired to measure transient values of energy, the low pass filter is eliminated. Also, lamp 312 then becomes a gaseous discharge lamp and means are provided to pulse the lamp, such as a Schmitt trigger circuit. With this type of operation the repetition frequency of the pulses determines the average illumination and so the feedback control elfect.

In airanalogous manner the next adjacent higher frequency increnent is filtered from the whole response of lnechanoelectric transducer 27 by filter 2, is amplified by amplifier A rectified, filtered, meter-indicated and attenuated by the other 2 entities and delivered via wire 55 as gain control energization to another lamp 12 (not shown) to be found in photomultiplier entity 2.

Similarly, channel 3' gives inverse feedback control for channel 3 and the next higher adjacent frequency increment; and so on to the total of n channels and Corresponding increments of higher and higher frequency. We have found that for channel 4- and higher, the active filter of the type of l is not required and that these filters may be of the passive type of direct channel filters 3 2; 3, Gill.

When vacuurri tubes are employed in the several filter channels a filament lamp 12 having a voltage rating within the range of 25 to 110 volts is suitable. When transistors are employed instead, the circuits are normally arranged for lower impedance throughout, and the voltage rating of the lamp is in the 4 to 20 volt range.

While the photomultiplier may have a wide range of characteristics and still be suitable for element 11, we prefer tie older types, such as the (RCA) type 931, 931A, etc. It will be realized that as long as the operation of these tubes is stable and generally reliable, the otherwise rejects may be employed.

The meters 45 in each of the several channels allow convenient determination of the spectral characteristic of an unknown type of vibration. The unknown vibration is caused to shake electromechanical transducer 23. The electrical output of mechanoelectric transducer 27 is thus a replica thereof. Alternately, if the electrical representation of the unknown noise or vibration is available, this is merely reproduced upon input bus 5'7 in lieu of an output from element 27 and the above processing repeated. Also, by observing the meters 49 the operator can observe the spectral behavior or the system under any type of operation and may alter the characteristic as desired.

On the other hand, meters 4% are not an integral part of the several feedback loops, and so can be dispensed with.

FIG. 3 shows an alternate embodiment of our inven- I tion, in which the randomness of the noise sources is provided by only two such sources and the servo control is accomplished by electrically adjustable attenuators.

Noise source 1 of PEG. 3 may be any having a true random electrical output. We have employed a commercially available gas-filled triode 6t) which is connected as a diode by connecting the cathode and the grid together. This has been the 6B4- tube. A magnetic field has been impressed across this tube by a magnet having poles 61 in order that the discharge in the tube shall not be oscillatory. Resistor 62 is connected to the anode of tube 6% and allows the noise output to be built-up as a voltage thereacross. Capacitor 63 is connected between the anode and the output terminal to remove the positive D.C. potential from that output, such potential being supplied by a source indicated by the plus sign but not shown. The other output is from the cathode of the tube and this is connected to ground. Throughout FIG. 3, when two conductive paths are not shown between block elements the conductive path through ground is'inferred.

The noise output from source 1 is fed to amplifier A which raises the level to a few volts, as did the same amplifier in FIG. 2.

Since there is not a separate noise source for each filter channel in this embodiment it is necessary to obtain servo (or automatic gain) control by other means. We prefer the electrically adjustable attenuator 1 having a small electric lamp 64 which alters the resistance of a photosensitive resistor 65 and which is connected in shunt to lter 1 channel. Resistor 66 is connected in series in the channel adjacent to resistor s5. Resistor 66 is not photoelectric, but is of the usual fixed type. In a manner of control analogous to that already described for FIG. 2 the brightness of lamp 64 provides constant gain for channel 1 for a given setting of desired attenuation therein. despite variation of secondary parameters.

The noise signal in channel 1 then passes to filter i and only the relatively narrow band passed by that filter is passed on. Filter 1 in FIG. 3 is the same filter l in FIG. 2, as are other entities Similarly identified in the two figures.

The resistive matrix of FIG. 3 differs from that of FIG. 2. It will be understood that while any of the series of frequency-adjacent band pass filters have response at frequencies in common upon the skirt portion of each, the response to electrical energy a whole band-pass "Romney-interval away is negligible. Thus, alternate filters of a frequency-adjacent group of filters can be directly connected to given matrix resistor and separate such resistors are not required.

lfn FIG. 3, therefore, the outputs of filters 1 and 3 connect directly to one terminal of resistor 67, while the outputs of filters 2 and 4 connect directly to resistor 6%, and so on for the n filters employed. The opposite ends of these resistors are connected to shunt resistor 69. The latter has a higher value than resistor 21 in FIG. 2, as two, three, or 12/2 or 11/3 times higher, depending upon Whether the individual filt rs are connected in alter-- nate pairs, whether a triple configuration, or an up to n/2 or 11/3 configuration following the same scheme is employed. This arrangement reduces the matrix loss aecordingly.

The power amplifier 22, electromechanical transducer 23, mechanoelectrical transducer 27 and the mechanically coactive element 23 between these two entities are the same as in FIG. 2.

Similarly, the feedback paths in FIG. 3 are the same as in FIG. 2; there being a primed path for each of the similarly numbered direct channels in both figures. These entities have been previously detailed and the permissible range of alternates set forth, so no further explanation is required here.

The point of difference between the two figures in this feedback portion of the apparatus lies in the placement of the feedback control lamp 64 in association with an attenuative device rather than as a means 12 to energize a noise source. Lamp as and photoresistor 65 may be combined in a small tubular light-tight enclosure and represent a elatively inexpensive means for attenuation control.

Also, this mode of control is small and simple as compared to other combinations, such as a reversible motoroperated potentiometer.

The lamp circuit from rectifier-meter l to lamp 64 is detailed by reference to FIG. 2 for the former entity and by noting wires 71, etc. in FlG. 3. The circuit of KG. 3 may be formed of components of somewhat different values and/ or characteristics in order to coact with the photoresistor rather than with the photomultiplier, as will be understood by those skilled in the art, but the method of operation is the same; a brighter light caused by greater feedback energy reduces the energy passed by the main filter channel. 1

The connections from the rectifier-meter entities in FIG. 3 to the lamps thereof as connections 71 for channel 1-1, are 72 for channel 2-2, "73 for channel 3-3, '74 for channel l4l', etc. to channel .'tll'.

FIG. 4 is illustrative of a further embodiment of our system in which the noise spectrum utilized from the wide band noise sources is at a higher frequency than is finally employed for useful purposes and after filtering into band pass increments the noise is heterodyned down in frequency to the frequency desired. PEG. 4 is fragmentary and is to be taken in Connection with FIG. 2 for completion. It will also be later appreciated that the mode of operation and the instrumentalities of FIG. 4 may also be employed in connection with the showing of FIG. 3. It is known that the noise sources of either of FIGS. 2 or 3 produce random noise well into the radio frequency spectrum. In fact, a low pass filter is sometimes employed with these sources in such embodiments in order that there shall not be spurious radio frequency energy passing as uncontrolled residuals throughout the systems. 111 P16. 4, however, use is made of what may be termed intermediate frequency noise according to the terminology of the radio receiver art. Should a useful spectrum from a subaudible lower limit to, say, 2,090 cycles as an upper limit be desired, an intermediate frequency spectrum of corresponding width is selected within the range of perhaps 10 to 300 lcilocycles. Where the intermediate frequency is chosen depends upon the type of filter elements it is desired to use and upon similar practical aspects.

Let it be assumed that the intermediate frequency band chosen is from 50 to 52 lcilccycles. 1F. Filter 1 in FIG. 4 then becomes a magnetostriction filter, say, having the narrow frequency band of from 50 to 50.025 kilocycles. LP. Filter 2 is the same, but with a frequency band of from 50.025 to 50.050, andso on. Magnetro-striction filters are known to have sharp resonances and so may be employed for such closely adjacent frequency bands. It will also be understood that an intermediate frequency of the order of 10 kilocyeles may be used should it be desired that the precision of filtering frequency be only one-fifth as great as for the 5f) kilocycle example.

As known, in magnetostrictive filtering a magnetostrictive tube, say of nickel, is energized to mechanical vibration by impressing the incoming electrical energy as a varying magnetic field in a coil surrounding the tube and taking off the filtered resultant with another coil.

Alternatcly, a. crystal lattice type of filter may be employed, in which quartz crystals are used. In this type of filter a number of such crystals are connected in a network to form a flat topped response bandpass filter with a good adjacent frequency attenuation characteristic.

The number of complete channels in FIG. 4 is the same as in FIGS. 2 or 3. The several filter outputs are brought together to a mixer tld at input fill. The mixer is the same as employed in the radio receiver art; a vacuum tube having one electrode of the nature of a grid upon] which the incoming signal energy is impressed, as at 81, and another electrode, as another grid or the cathode, upon which heterodyn-ing oscillator electrical energy is impressed. This comes from oscillator 82. As is also known, the difference in frequency between that of the oscillator and the intermediate frequency is chosen so that that difference is the desired useful frequency. For filter channel 3 this is of the order of 109 cycles. It will be understood that at shunt resistor 21 in PEG. 4, the same spectrum of incremental frequency bands having individually determined amplitudes of noise energy as present at resistor'Zl in PEG. 2, is present. Accordingly, the remainder of the structure of FIG. 2 is included in FIG. 4 by inference. The individual feedback conductors, such as 55 in FIG. 4 are the same as shown in FIG. 2 and indicate the return of each feedback loop to the initial source for the channel.

in the operation of any of the systems of this type it is to be noted in passing that the aspect of phase cancellation has greater practical si nificance than phase reinforcement. What this means is that phase distortion at the skirts of frequency-adjacent filters may cause sufficient cancellation by algebraic addition of opposed components to place a deep out, or null, in the amplitude vs. fre quency characteristic of the random noise spectrum, but that reinforcement can only cause a peak of 3 db, or

iii

double power. As has been previously mentioned the actual detail of such algebraic addition is far too fortuitous to be controlled by engineering design and the only answer is to employ separate sources of noise for adjacent frequeney bands according to our invention.

t will be understood that certain modifications may be made in the illustrative embodiments described herein.

The photomultiplier light-source type of noise generator of FIG. 2 may be employed in the apparatus of FIG. 3 instead of the gas-tube source. In such a modification the lamp 12 of the light-source generator is maintained at strictly uniform illumination, since this then determines the noise level for many filter channels. Servo control is obtained by lamps, as 64, in the several attenuators of FIG. 3.

The two noise sources shown in FIG. 3 represent the minimum number of such entities in that type of embodi ment. A greater number, such as four, six, etc. may be employed, with the various filter channels grouped to be fed from different sources. Our criterion that filter channels having frequencies in common must be fed from separate sources must still be observed.

Theresistive matrix of FIG. 3 may be employed in FIG. 2 and vice versa.

Othermechano-resonant elements may be employed for the LP. amplifiers of FIG. 4, such as tourmaline or Rochelle salt.

The order of connection in the series circuits of the filter channels is subject to some alteration. The positions of amplifier A and filter 1 may be interchanged in FIG. 2, and so also for the other channels 2, 3, etc. A similar interchange may also be made in FIG. 3 with respect to amplifier A and filter 1, etc.

Still other modifications in the characteristics of the cir cuit elements, details of circuit connections and alteration of the coactive relation between the elements may be taken without departing from the scope of our invent-ion.

Having thus fully described our invention and the manner in which it is to be practiced, we claim:

1. In an electromechanical system for producing random vibration having an electromechanical transducer and a mechanoelectric transducer actuated thereby, electrical circuit means connected between said transducers comprising plural frequency-selective filters having adjacent frequency pass bands with certain frequencies in common therebetween, at least two separate sources of random electrical noise, said sources connected to said filters having adjacent frequency pass. bands so that said electrical noise comes from separate said sources at each of said frequencies in common, all of said filters connected to said electromechanical transducer, the same plurality of further frequency-selective filters having adjacent frequency pass bands with the same said certain frequencies in common therebetween, and servo-control means connecting each of said further filters with each of said firstmentioned filters having the'same pass band for the amplitude control of said random electrical noise in the circuit of the pair of said filters.

2. In an electromechanical system having an electromechanical transducer and a mechanoelectric transducer coactive with said said electromechanical transducer, a feedback controlled energizing system comprising plural sources of random electrical signals, a first group of plural frequency-selective filters connected to said plural sources to form plural frequency-adjacent direct channels with each channel having randomly related electrical energy signals in relation to the signals in the adjacent channel, all of said direct channels connected to said electromechanical transducer, a second group of plural fre quency-selective filters connected to said mechanoelecttic transducer to form plural frequency-adjacent feedback channels, adjustable means to selectively attenuate said signal level in each of said feedback channels connected thereinto, and means connecting each said feedback channel to each said direct channel having the same frequency range to control the signal level in each said direct channel inversely according to the attenuation of said means-toattenuate in the corresponding said feedback channel.

3. The feedback-controlled energizing system of claim 2 in which said means connecting each said feedback channel to said direct channel having the same frequency range is an electric lamp, and further in which each of said plural sources has a photosensitive element, and each said electric lamp is disposed to illuminate the photosensitive element of the said direct channel of frequency corresponding with that of said feedback channel.

4. The feedback-controllcd energizing system of claim 2 in which said means connecting each said feedback channel to said direct channel having the same frequency range is an electric lamp, and further in which each said direct channel has a photosensitive resistor, and each said electric lamp is disposed to illuminate said photosensitive resistor of the said direct channel of frequency corresponding with that of said feedback channel.

5. In an electromechanical system having an electromechanical transducer, amplifier means for energizing the same connected thereto, and a mechanoelectrical transducer actuated by said electromechanical transducer, an automatically-controlled energizing system comprising plural sources of random electrical noise signals, a first group ofplural band-pass frequency-selective filters each thereof connected to one of said plural sources to form plural frequency-adjacent direct channels each having noncoherently "elated electrical energy signals, the outputs of all of said direct channels connected in parallel to said amplifier means, the inputs of a second group of similar plural band-pass frequency-selective filters all connected in parallel to said mechanoelectric transducer to form plural frequenc -adjacent feedback channels, rectifier means connected in each of said feedback channels, means to indicate the signal level in each of said feedback channels connected thereinto, adjustable means to selectively attenuate said signal level in each of said feedback chan nels connected thereinto, and electro-optical means connecting each said feedback channel to the said direct channel that has the same frequency range to control the signal level in each said direct channel inversely according to the attenuation selected for said means-to-attenuate in the corresponding said feedback channel.

6. In an electromechanical system having an electromechanical transducer and a mechanoelectric transducer coactive with said electromechanical transducer, a feedback controlled energizing system comprising plural individual sources of electrical noise signals each having amplitude control means, a filter connected to each said source, the frequency characteristic of each said filter being essentially unique and in the sum to pass a contiguous frequency spectrum, each of said filters connected to said electromechanical transducer, plural further filters each having the same frequency characteristic as one of the first mentioned filters, said plural further filters connected to said mechanoelectric transducer for energization thereby, means to rectify connected to each of said further filters, and an attenuator connected to each said means to rectify, each of said amplitude control means connected to the said attenuator that is connected to said further filter having the same frequency characteristic as that of a first mentioned said filter to thereby close each unique loop of said feedback-controlled energizing system.

7. In an electromechanical system having an electromechanical transducer, electrical power means for energizing the same connected thereto, and a mechanoelectrical transducer actuated by said electromechanical transducer, a servo-controlled multi-channel energizing system comprising plural individual photomultiplier sources of electrical noise signals each having illumination means of intensity that can be varied, a band pass filter connected to each said source, the passed band of each being essentially unique and in the sum to pass the whole of a desired frequency spectrum, each of said band pass filters connected to said power means, a group of band pass filters each having a pass band the same as one of the previously mentioned band pass filters, said group connected to said mechanoelectric transducer for energization thereby, a rectifier connected to each band pass filter of said group, a low pass filter connected to each said rectifier, a meter connected to each said low pass filter, and a variable attenuator also connected to each said low pass filter, each of said illumination means connected for variation of intensity to the servo channel attenuator containing that band pass filter of said group that passes the same passed band as the previously mentioned said pass band filter which is connected to the photomultiplier that said illumination means illuminates.

8. in an electromechanical system having an electro mechanical transducer and a mechanoelectrical transducer coactive with said electromechanical transducer, a feedback-controlled energizing system comprising plural separate electrical sources of random noise signals, plural attenuators each having means for varying the attenuation thereof, a first group of plural frequency-selective filters having frequency-adjacent passed bands, one of said first group of filters connected to each one of said plural attenuators, said sources connected through said attenuators to said first group of filters in alternate manner such that electrical energy present at every frequency common to any two filters is supplied by separate said sources, all of said first group of filters connected to said electromechanical transducer for the composite excitation thereof; a second group of plural band-pass frequency-selective filters having the same individual frequency characteristics as those of said 1Si'. group and similarly arranged as to common frequencies, said second group of filters connected to said mechanoelectrical transducer for energization thereby, a rectifier-filter connected to each of said second group of filters, and a manually adjustable variable attenuator connected to each of said rectifierfilters; each of said manually adjustable attenuators connected to the corresponding electrical means for the control of the one said plural attenuators associated therewith to provide ne ative feedback control of the multifrequency electrical energy flowing in said system for maintaining the signal level of each passed frequency band proportional to the attenuation of each said manually adjustable attenuator.

9. In an electromechanical system having an electromechanical transducer, electrical power means for energizing the same connected thereto, and a mechanoelectrical transducer actuated by said electromechanical transducer, a servo-controlled energizing system comprising two separate sources of electrical random noise signals, a first group of plural band-pass frequency-selective filters having frequency-adjacent passed bands, a group of electrical attenuators having electro optical means for varying the attenuation thereof, said sources connected to said first group of filters through said electrical attenuators in an alternate manner such that electrical energy present at every frequency common to two filters is supplied by one of said two sources to one of said two filters and by the other of said two sources to the other of said two filters, each of said first group of filters connected to said power means for the composite excitation thereof; a second group of plural band-pass frequency-selective filters having the same individual frequency characteristics as those of said first group and similarly arranged as to common frequencies, said second group of filters connected to said mechanoelectrical transducer for excitation thereby, a rectifier connected to each of said second group of filters, a low pass filter connected to each said rectifier to integrate the response thereof, means to indicate signal level connected to each said low pass filter, and a manually adjustable attenuator also connected to each said low pass filter and to the corresponding electrooptical means for the control of the said electrical attenul3 ator associated therewith to provide a negative feedback type of servo control of the multifrequency electrical energy flowing in said system for maintaining the signal level of each passed frequency band at a level determined by the attenuation setting of each said manually adjustable attenuator.

10. In an electromechanical system having an electromechanical transducer, and a mechanoelectrical transducer coactive with said electromechanical transducer, and automatic gain control multichannel apparatus for controlling said system comprising plural sources of electrical noise each having gain control means, a band filter connected to each said source of noise, means-to-heterodyne said electrical noise passed by each said band filter to a lower frequency, said means-to-heterodyne con nected to each of said band filters and to said electromechanical transducer; a group of band pass filters connected to said mechanoelectrical transducer, one of each said group having a pass band corresponding to each said lower frequency, rectifier means connected to each filter of said group, and attenuator means connected to each said rectifier means and to said gain control means for the one of said sources of electrical noise corresponding in said lower frequency to the frequency of said band filter to which said attenuator means is connected for controlling the gain of each feedback loop thereby formed.

11. In an electromechanical system having an electromechanical transducer and a mechanoelectrical transducer coactive with said electromechanical transducer, an automatic gain control multichannel apparatus for controlling said system comprising plural sources of electrical noise, a group of first filters having frequency-adjacent passed bands, plural attenuators each having means for varying the attenuation thereof, each one of said group of first filters connected to one of said plural attenuators to form plural pairs of these elements, said plural attenuators connected to said sources in an alternate manner such that electrical energy present at every frequency common to any two said first filters is supplied by different said sources, means-to-heterodyne said electrical noise passed by each said first filter to a lower frequency, said means-to-heterodyne connected to each of said first filters and to said electromechanical transducer; a group of sec-- ond filters connected to said mechanoelectrical trans ducer, one of said second filters having a pass band corresponding to each said lower frequency, a rectifier connected to each said second filter, each said rectifier connected to said means-for-varyingattenuation corresponding in said lower frequency to the frequency of said second filter to which said rectifier is connected for controlling the gain of each feedback loop thereby formed.

12. In an electromechanical system having an electromechanical transducer, electrical power means for energizing the same connected thereto, and a mechanoelectri-- cal transducer actuated by said electromechanical transducer, an automatic gain control multichannel apparatus for controlling said system comprising plural individual sources of electrical noise each having gain control means, a narrow band filter having mechano-resonant elements connected to each said individual source of noise, an electrical oscillator operative at a frequency higher than each of the resonant frequencies of said narrow band filter, a mixer connected to said oscillator and to each of said narrow band filters to supply a hetrodyned output at a frequency lower but proportionally related to each of said resonant frequencies, said mixer connected to said power means for the composite energization of said electromechanical transducer; a group of band pass filters connected to said mechanoelectrical transducer, each having a pass band corresponding to the frequencies of said heterodyned output contributed by one of said narrow band filters, a rectifier connected to each filter of said group, and an attenuator connected to each said rectifier and to said gain control means for the one of said sources of electrical noise corresponding in output fre- M quency from said mixer to the frequency of said band pass filter to which said attenuator is connected for the gain control of the feedback loop thereby formed.

13. In an electromechanical system having an electromechanical transducer and a mechanoelectrical transducer actuated by said electromechanical transducer, an auto-' nratic apparatus for controlling said system comprising plural individual sources of electrical noise, a group of first narrow band resonant filters having adjacent frequency passed bands, plural variable attenuators, each of said group of first filters connected to one of said plural attenuators to form plural pairs of these elements, said plural attenuators connected to said sources of electrical noise such that electrical energy present at every frequency common to any two said first filters is supplied by different said sources, an electrical oscillator operative at a frequency higher than each of the resonant frequencies of said first filters, a mixer connected to said oscillator and to each of said first filters to supply a heterodyned output at frequencies lower but proportionally related to each of said resonant frequencies, said mixer connected to said electromechanical transducer for energization thereof; a group of second filters connected to said mechanoelectrical transducer, each said second filter having a pass band corresponding to each said lower frequency, a rectifier connected to each said second filter, each said rectifier also connected to said variable attenuator corresponding in lower frequency to the frequency of said second filter to which said rectifier is connected for controlling the gain of the feedback loop thereby formed.

14-. In an electromechanical system with adjustable amplitude response as a function of frequency having an electromechanical transducer and a mechanoelectric transducer coactivc therewith, plural electrical means devoid of phase cancellation over a frequency spectrum connecting said transducers in plural separate feedback loops comprising plural frequency-selective filters having certain frequencies in common therebetween, plural separate equivalent sources of random electrical noise, separate said sources connected to those filters having frequencies in common so that said electrical noise is random at said common frequencies, and plural separate feedback control means connected between corresponding filters and equivalent sources of noise to provide feedback control of the noise output of said sources.

15. In an electromechanical system for producing random vibration having an electromechanical transducer and a mechanoelectric transducer actuated thereby, electrical circuit means connected between said transducers to form plural feedback loops comprising first and second sets of plural frequency-selective attenuation-adjustable filters having adjacent frequency pass bands with certain frequencies in common therebetween, and at least two separate sources of like random electrical noise, separate said sources connected to said first set of filters having adjacent frequency pass bands so that said electrical noise is random at all adjusted amplitudes of said frequencies in common, a set of means-to-control the amplitude of said noise in each of said adjacent frequency pass bands, said set of means-to-control connected to said first set of filters to exercise control in each of said feedback loops, and said set of means-to-control also connected to said second set of filters for adjustment of said control by the noise energy passed by said second set of filters in. each corresponding said feedback loop.

References Cited in the file of this patent UNITED STATES PATENTS 2,153,528 Benning Apr. 4, 1939 2,413,066 Parington Dec. 24, 1946 2,686,256 Albersheim Aug. 10, 1954 2,697,760 Schlenker Dec. 21, 1954 2,935,672 Ross May 3, 1960 2,953,786 Goldfischer Sept. 20, 1960 2,974,198 McLeod Mar. 7, 1961 

14. IN AN ELECTROMECHANICAL SYSTEM WITH ADJUSTABLE AMPLITUDE RESPONSE AS A FUNCTION OF FREQUENCY HAVING AN ELECTROMECHANICAL TRANSDUCER AND A MECHANOELECTRIC TRANSDUCER COACTIVE THEREWITH, PLURAL ELECTRICAL MEANS DEVOID OF PHASE CANCELLATION OVER A FREQUENCY SPECTRUM CONNECTING SAID TRANSDURCERS IN PLURAL SEPARATE FEEDBACK LOOPS COMPRISING PLURAL FREQUENCY-SELECTIVE FILTERS HAVING CERTAIN FREQUENCIES IN COMMON THEREBETWEEN, PLURAL SEPARATE EQUIVALENT SOURCES OF RANDOM ELECTRICAL NOISE, SEPARATE SAID SOURCES CONNECTED TO THOSE FILTERS HAVING FREQUENCIES IN COMMON SO THAT SAID ELECTRICAL NOISE IS RANDOM AT SAID COMMON FREQUENCIES, AND PLURAL SEPARATE FEEDBACK CONTROL MEANS CONNECTED BETWEEN CORRESPONDING FILTERS AND EQUIVALENT SOURCES OF NOISE TO PROVIDE FEEDBACK CONTROL OF THE NOISE OUTPUT OF SAID SOURCES. 